Production of alkali metal tubes



Patented June 7, 1932 UNITED STATES PATENT OFFICE .i JOHN W. MARDEN AND ERNEST A. LEDERER, OF EAST ORANGE, NEW JERSEY,

SIGNORS TO WESTINGHO'USE LAMP COMPANY, A CORPORATION 0F PENN SYLVANIA PRODUCTION OF ALXALI METAL TUBES Application tiled October 27, 1927. Serial No. 228,994.

This invention relates to a vacuum electric device employing a thermionically active cathode and more particularly to such a device in which thermal emission is imparted to 5 the cathode by the presence of an alkali metal vapor such as caesium, rubidium or the like. l

It has been found that the alkali metals under proper conditions greatly increase the 10 electron emission of a refractory metal cathode such as a tungsten cathode in a thermionic device so that high electron emissivity is imparted thereto at a temperature so low that in the absence of such alkali metal vapor no substantial emission would occur. This i phenomenon is explained on the theory that a film of the alkali metal forms continuously on the refractory metal body, as for instance a filament, which film resists evaporation at temperatures much in excess of the vaporization temperature of thealkali metal when the refractory cathode consists of a suitable netal such as tungsten or molybdenum.

The formation of the alkali metal film, preferably caesium, is greatly facilitated by the presence of an electro-negative gas on the surface of the refractory body. Oxygen has been most successfully used in this connection and when an oxide or au oxygen film is produced on the surface of the tungsten or molybdenum body it renders this body capable of holding on .to or causing the alkali metal atoms to adhere thereto. more strongly than does the plain surface of the tungsten or molybdenum electrode.

In the construction of devices of this nature. diiiiculty has been encountered due to the sensitivity of this active film of' alkali `metal which serves as the electron-emitting material and to the ease with which the oxide or oxygen layer produced on the tungsten or molybdenum is destroyed. as b v the presence of hydrocarbon or other reducing materials in the device. The alkali metals are extremely active and if the proper precautions and methods of construction are not carefully y followed, the alkali metal will completely combine with impurities, etc., and will not be efl'lective for accomplishing the desired resu ts.

cathodes in which the alkali metal will be p maintained in a highly active state.

A still further object is to provide a. method of producing an electron discharge device employing a thermionically active cathode and of producing and maintaining on said cathode an active film of an alkali metal.

A further object is to provide a method of rendering a refractory metal bod thermionically active by the formation t ereon'of an active deposit of an alkali metal such as caesium.

Other objects and advantages will hereinafter appear.

In order that this invention may be more fully understood, reference will be had to the accompanying drawing in which:

Fig. 1 illustrates, partly in section, an electron discharge device of the hot cathode type employing an alkali metal for imparting electron emission to the cathode thereof;

Fig. 2 is an end viewvof the plate and control electrode structure shown in Fig. 1; and,

Fig. 3 is a diagrammatic illustration of the tube shown in Fig. 1 in connection with the gas supply and exhaust connections employed in the manufacture thereof.

The discharge device shown in the figures includes a bulb 10 having a flarev tube 11 provided with the usual press 12. Extending from the press are a plurality of support-members for securin, lr the several electrodes in position as will be more' clearly hereinafter described.

The elet-trod@ assembly includes a filament 13. a control electrode or grid 14 and a plate electrode or :mo-f2. rrhh-h is designated as a whole by the numeral 15 an? which is composed of two plate-mennen; 1G and 1T disposed on opposite sides of the control electrode 14. The electrodes are s-ecred to the support wires extending from the press and constitute the' mount 18. The flare tube 11 having the mount 18 thereon is sealed to the bulb 10 at the juncture 19 and the bulb may bey exhausted through an appropriate exhaust tube 20 which extends from the interior of the flare tube.

Each of the plate members 16 and 17 .of the plate electrode or anode 15 is composed of a ladder-like structure having a plurality of horizontal rungs or vanes 21 in the form of fiat metallic strips struck out from a plain sheet of metal and connected at their ends to support-members 22 and 23 (see Figure 2) integral therewith. The vanes 21 are arranged in parallel spaced relation and in planes substantially transverse to the support-members 22 and 23 so thatwhen the plate-members are in position the edges of the vanes will be positioned toward the lilament.

The grid-member 14 includes a set of vanes.

24 and 25 connected. to support-members 26, 27 and 28, 29' respectively, arranged so as to hold the vanes in spaced relation to form a lattice-like enclosure.

The vanes 21 of the plate and the vanes 24 and 25 of the control electrode or grid are disposed in spaced overlapping relation. By reason of thls construction, the ilament or cathode is rotected from positive ion bombardment ue to the electric field produced between the vanes of the respective electrodes.

The lament 13 is of Ms-shape and is connected at its ends to arms 30 and 31 secured to a common support-member 32, sealed in the press 12 and connected to current conductor 33. The central loop of the filament is connected to a support-member 34 also sealed in the press and terminating in a conductor 35.- Theupper loops of the filament are engagedb resilient supports secured to arms 36 an 37 extending from a glass bead 38. This bead is secured to an upright member 39 attached to a support 40 sealed in the press. The supports40 is connected to a conductor 41 for supplying current to the plate electrode 15 through the rod 42.

The grid 14 is secured to supports 43 and 44 which extend upwardly through the press 12and are welded to the side members 26 and 29.

In order to provide a supply of alkali metal in the device, a pellet 45 which may be composed of a compoundof 'the alkali metal and a'reducing agent is supported by a loop 46 in the metallic strip 47 secured to a .suitable support as, for instance, the grid support 43. The pellet is supported in a loop 1n such manner that it may be readily heated by high frequency induction current todecompose the alkali metal compound and liberate the alkali metal as will be more fully brought out hereinafter.

The mechanical construction of thisdevice forms the subject matter of a co-pending application of the No. 228,995, filed ctober 26, 1927, and en-4 titled Electron emission device.

The present inventionv is concerned more particularly with the procedure involved in 'the manufacture of the apparatus described and in accordance therewith, we prefer to thoroughlyy clean the anode and control electrode plates after they vare formed and before mounting to remove as far as possible the oxygen, oxides, grease, impurities, etc., thereon by baking these parts in hydrogen gas for a suiicient period, usually 5 to 10 minutes, at a relatively high temperature, that is, around 1000 C. The cleaned and deoxidized plates and grid parts are then mounted together with the tungsten or molybdenum filament 13 upon a previously formedl press. After mounting, if the ilament has not been previously cleaned, it should be cleaned and straightened by ashing the same in an inert gas, such as forming gas, (composed of hydrogen and'nitrogen, the latter being in the proportion of about for a short time, that is, from four to live seconds, at a temperature of about 1400o C. This step has been readily produced thereon in a subsequent operation.

During the handling and mounting of the plates and grids, these members become more or less contaminated with grease from the fingers, etc. It has been found desirable to bake the completed mount at this point of the procedure for 30 minutes to one hour at about 300 C. in the air. This may be omitted but it has been found that accumulated grease and dirt are very largely removed in this manner either by' volatillzation or oxidation.

We next prefer to coat the press 12 and the glass bead 38 in which conductingl wires are sealed with a material which serves to 'prevent electrical conduction between the elementsof the tube. This electrical conduction usually 'called leakage is due to a de posit of the alkali metal on the press and the titled Brevention of electrical leakage.

Briefly, this process consists in slowly heatmg ing a pressure of one to two cm. which procedure removes the volatile gases and vapor.-

izable impurities. A solution Aof 1 to 10% of this purified petrolatum residue in chloroform is made and is applied to the pressand commercial petrolatum up tb about 380 .to 400 C.'for one-half hour, while maintainresent inventors, Serial`r tion through the exhaust `or to about 220 to 250 bead in any convenient manner as by painting, spraying or the like.

In lieu of the petrolatum coating, we may use a coating of shellac or other resinous material as set forth in co-pending application of Emil G. Widell, Serial No. 30,662, filed May 16, 1925, and entitled Electron discharge device. However, in using shellac, greater precaution must be taken during the exhaust operation and we prefer to employ the petrolatum solution as the leakage preventor.

After the complete mount has been sealed into the bulb 10, the same maybe exhausted connection 48 (see Fig. 3). The bulb in the meantime, being baked to a temperature of about 300 C. when employing petrolatum on the press and bead C. when employing shellac on these glass parts. The exhaust process is, of course, continued until no further gas comes from the envelope, but ordinarily should be completed in about ten minutes, depending upon the pumps employed.

It should be understood that the temperature and time oft-baking are inter-dependent. For exam le, baking for ten minutes at 300 C. gives a out the same degree of exhaust as baking at 400 C. for three to four minutes provided that the vacuum pumps employed are rapid enoughto remove the gases during a shorter higher temperature treatment. In actual factory practice the higher tempera ture more rapid exhaust is to be preferred.

Since oxidizing conditions should, as far as possible, always prevail during the exhaust of the bulb, it is essential to have the grease baked out of the bulb and driven far enough away from the place where the exhaust tube is sealed off so that this grease is not vaporized back into the bulb during the sealing ofi' process. In order to accomplish this, We prefer rather long exhaust tubes on the bulbs and to place the bulbs so that they are well up in the exhaust oven.

After exhaustion, the plate and grid electrodes are heated to as high a temperature as they may stand by high frequency induccurrent to drive out any gas contained therein. In heatin these elements, however, they must not be subjected to a sufficient temperature to cause the press or the bead to attain an excessive temperature. The high frequency induction 'coil employed for treating out these electrodes is indicated at 49 Ain the position in which it should be employed with the construction illustrated.

v Referring, for instance, vto Figure 2, it will be noted that the grid electrode forms in plan view a complete loop, the two opposed sides being joined throughout their length by the integral end portions 26, 27 and 28, 29, thus the grid becomes highly heated by the higll frequency inductioncurrent directly by induction and the plate electrode which is arranged relatively close thereto becomes heated by radiation. The gases liberated by this high frequency treatment are removed by the dependent upon the proper formation of such layer of electro-negative gas.

In carrying out this step, we admit oxygen through the-stop-cock in the connection 50 at such pressure asrto make 5 to 50 cubic mm. at atmospheric pressure available for the oxidation of the filament. The exact pressure depends upon the volume of the exhaust manifold andupon the number of tubes on the manifold. In any case, although the exact pressure of oxygen required by a filament is not 'very critical, we have found that best activation is obtained when 5 to 50 cubic mln. are allowed for each filament. Higher pressures can be used but due to excessive oxidation of the tungsten filament it is difiicult to get predetermined filament voltages and currents under these conditions.

These conditions are suitable for the production of a tube having filament characteristics on operation of 1 volt and 60 ma. The filament Weight in this case is about l1.5 to 2.0 m. g. per 200 mm. length. In case very much larger tungsten filament is used, larger quantities of oxygen are required.

The filament is flashed in this oxygen atmosphere at a temperature of around 2000 C. for about one-half minute. The effect of this high temperature flashing is to clean the filament thoroughly of any grease or hydrocarbons or other impurities which might otherwise react with the oxygen or oxide layer formed thereon. The filament after this cleaningl operation becomes oxidized to a sulicient extent to cause the alkali metal which is subsequentlyl liberated to adhere thereto very much more tenaciously than it would to the unoxidized filament.

If desired, the filament may be heated in vacuum and the oxygen admitted after such heating, inasmuchas the high temperature is not required for oxidation but merely for cleaning. In place of pure oxygen, ordinary atmospheric air may be used, but in such case, it is necessary to employ a pressure of from three to four times that for oxygen. A still further means for providing an oxygen supply for the filament may be by means of an appropriate compound of the alkali metal, which upon being heated, will liberate oxygen, as will more fully appear in connection with the step of flashing the alkali metal pellet 45. l

,Where only a small amount of alkali metal is introduced, it is sometimes desirable to permit the metal parts to cool after the high frequency' treatment thereof before 4 the oxlylgen is admitted, since as these parts are st' at a high temperature they will readily oxidize and during the subsequent operation or during the life o f the device this oxide on the grid and plate combines with the alkali metal and destroys its effectiveness for rendering the filament thermionically active. Wehavefound, however, that one minute permits sufficient cooling to prevent such action. i

After the oxidation of the filament, the oxygen or air is pumped from theV device and if desired, in order to insure the removal of any hydrocarbons or other impurities which may have been liberated from the filament during the fiashing, the bulb may be flushed again with oxygen at substantially the same pressure. It is very essential to remove all reducing material from the bulb in order to protect the oxide layer on the filament. The above fiashing is merely a precautionary measure to insure the retention of the oxide layer on the cathode.

After the complete removal of the oxygen from the bulb, we proceed to introduce the desired alkali metal, preferably caesium by heating the pellet f5-as by high frequency induction to decompose the alkali metal compound in the envelope. In heating the pellet 45, it will be noted that it has been disposed in such manner that the closed loop of which the metal strip 47 forms a part is arranged at right angles to the loop formed by the rid electrode so that the high frequency in uction c oil should be arranged with its axis substantially normal to the side wall of the bulb for the heating of this getter material. This is desirable, since care must be taken not to reheat the metal parts during the fiashing` of the getter at a sufficiently high temperature to drive any additional gas therefrom which might attack the oxide or oxygen layer formed on the lament. The composition ofthe pellet 45 which we have found to produce the best results is a mixture of caesium permanganate and silicon as the reducing agent therefor.

The caesium permanganate and silicon mixture may be employed as a source of oxygen for oxidizing the filament and in such case, instead of Iadmittin oxygen to the bulb from an external source, t e pumps are sealed -off as by the valve 51 and the pellet heated to a low temperature to evolve oxygen therefrom by be lowered sol the decomposition of the caesium permangailate. After the desired gas pressure has been developed by the evolution of oxygen in this manner, the heating may continue. at a high temperature to cause the csium compound to react with the silicon to liberate caesium metal; h

e temperature required for 's around 900 C. but may hat by the addition of this decompos aluminum or slmilar'material to the mixture. The oxygen liberated by'thegetter is removed by the pumps after which the tube 'is sealed off and based.

Itis necessary to now activate the filament, to impart thereto the electron emitting properties of the alkali metal. This activation We prefer to conduct in two stages, a preliminary activation being first effected to render the filament sufficient thermionically active to permit seasoning of the tube to be accomplished to ionize and clean up residual gas. The final activation is then produced.

In order to produce the preliminary activation, Vive heat the filament with orv Without any otentialY on the gid'electrode or on the ano e and' with an excess of filament current over that for which the filament is normally designed. For example, with a fila- Iment designed for a 50 ma. normal filament current We flash the filament for about l0 l 'onthe grid in order to prevent positive ion bombardment of the filament which might destroy the caesium layer formed on the cathode.

The final activation is then effected by flashing the filament for about 20 seconds at 170 to 200 ma. filament current, preferably with no voltage applied between the electrodes. This final activation completes the formation of the caesium lm on the cathode and imparts thereto high electron emitting properties. While We have outlined a specific order for carrying out the various steps, it is to be understood that many changes may be made therein and in the details of many of these steps without departing from the invention and we do not desire to be limited to the specific details described.

What is claimed is:

1. The method of producing a thermionic -discharge device employing an alkali metal uum to a high temperature to drive the gases therefrom, heating the filament in an atmosphere of oxygen introducing an allialinmetal into the envelope andheating saidflament inthe presence of said alkali/metal vapor to cause'said alkali metal/t6 'adhere thereto.

2. The method of activating refractory metal filaments in a closed container which consists in heating said filament to a high temperature in oxygen at substantially atmospheric pressure, atmosphere and creating an atmosphere of an alkali metal vapor, heating said filament to about the normal temperature thereof in said vapor to cause the alkali metal to effect a partial activation of said filament, ionizing the residual atmosphere in said container and cleaning u the same and subsequently heating said fi ament to above the normal operating tem erature to complete the activation thereo 3. The method of activating a refractory metal filament in an electron discharge device Vhaving a cooperating electrode which comprises oxidizing said filament, generating an alkali metal vapor in said envelope, heating said filament in said vapor to partially activate the same, creating a discharge between said filament and cooperating electrode to ionize the gas in the envelope to clean up the same and subsequently heating said filament to an abnormal temperature to 4complete the activation thereof.

4. The method of producing a thermionic discharge device employing an alkali metal as the active electron-emitting material comprising cleaning the electrodes under oxidizing conditions, sealing the same into an envelope, heatingthe filament to a high temperature in an atmosphere of oxygen, removing said oxygen, introducing an alkali metal into the envelope and heating said filament to a high temperature in said alkali lmetal vapor to impart thereto the electron emissivity of the alkali metal.

5. The method of activating a refractory metal filament having an oxidized surface in a closed. container which consists in heating said filament to a high temperature in the presence of an alkali metal vapor to partially activate the same, ionizing the residual gaseous content of said container and' cleaning up the same by said alkali metal and subsequently heating the filament to a high temperature to complete the activation thereof.

6. The method of producing a thermionic discharge device employing an alkali metal as the active electron-emitting material, comprising assembling the electrodes of the device upon a mount, heating the said mount under, oxidizing conditions at a `temperature of abbut 300 C. for from 30 to 60 minutes, sealing said mount into an envelope, introducing oxygen into said envelope, heating the filament to about 2000 C. in said oxygen, kremoving said oxygen, generating an alkali metal in the envelope, heating said filament .-tofa temperature o fqaround 14000 C. for a few seconds in an inert orreducing atmosphere, heating said electfoies therein to a removing said oxygen su high temperature to degasify the same, and heating said filament to an abnormal temperature in said alkali metal.

In testimony whereof, we have hereunto ber, 1927.

bscribed our names this 26th day of Octo- JOHN W. MARDEN. ERNEST A. LEDERER. 

